Double-yolk eggs can be visually distinguished by their size. If you adjust to a sufficiently high false-negative rate (i.e. let enough of the double-yolked ones get into the regular stock), you can achieve a very low false-positive rate (i.e. you can very reliably pick out eggs that are double-yolked).

The grocery around the corner from me often sells flats of 15 double-yolked eggs. I usually buy those when they have them, and I don’t think I’ve ever found a single-yolk egg in one of those flats.